Elevator Hinge Reinforcement

[JG3]

Awhile ago Bill Grieve mentioned that someone had noticed a missing rivet head on the Elevator Hinge Reinforcement, part # SD025, and a couple of other aircraft were found with the black stains around the rivet heads that indicates movement.... I checked my aircraft and found all three rivet heads missing altogether, allowing the elevator to wobble sideways about 20mm! The purpose of that bracket is precisely to stop sideways movement of the elevator. The movement didn't show up in pre-flights because I just worked the elevator up and down for free movement but didn't push-pull sideways....

 

 

You can see those rivets by peering upward from under the right side of the elevator. There are three rivets in a horizontal row. It wasn't difficult to replace the rivets. Just needed to cut away a small tab of the skin on the underside of the elevator, and needed a close fit right-angle drill to drill out the old rivet cores. Then could just reach in with a slim model riveter. I used 3/16" stainless rivets time, and will be keeping a close watch on them and their supporting structure.

 

Can't figure how those rivets could have felt the stress needed to break them..... But also checked a Zenith 701, which is identical construction, and found one rivet sheared, so it's a common issue. Everyone with Savannah and 701 aircraft need to keep a close eye on those rivets..... I don't know if anyone has informed ICP yet, but there should be an AD in that regard.

 

JG

 

 

[IBob]

Thanks JG.

 

I'm a novice, building a Savannah S, so I'm very interested.

 

I just finished fitting plastic tips, which required the temporary assembly of Fin to Elevator. Without that centre hinge bolted, the outer 2 hinges are almost like two springs: the elevator can twang freely from side to side. Perhaps, if there is some small clearance in the centre hinge, the elevator constantly vibrates from side to side, eventually fracturing the rivets.

 

Tomorrow I will set up the centre hinge by the manual, and see what clearance that gives.

 

It also occurs to me that with all 3 rivets gone, the brass bushing and associated holes in the brackets may now be flogged out some. I'm sure you already thought of that (too).

 

Bob

 

 

[IBob]

For what it's worth, the centre hinge on my S can be set up, by the book, with little or no clearance. In fact you could probably discard one of the two RH washers. The drawing itself is not quite correct, as the Stabiliser is two thicknesses at that point, being SD024 and SD025 riveted together.

 

 

 

 

[rankamateur]

Maybe two or three of the rivets could be replaced with bolts for peace of mind.

 

 

[IBob]

Hindsight being 20/20, you can see with that setup that most of the lateral load will be taken by the end rivet, rather than shared between them...still, it's not easy to understand how there could be that much lateral load.

 

Since Facthunter (I think) recently pointed out that you can fly without roll and yaw, but you cannot fly without pitch, I find myself very focused on this elevator!

 

 

[Nobody]

Maybe two or three of the rivets could be replaced with bolts for peace of mind.

Or you could use a solid rivet with its higher strength....

 

 

[IBob]

I would think a first step is to ensure that the central pivot is set up to be free of lateral movement or slop. It would be interesting to know if aircraft where this has occurred are set up this way; also the condition and slop of trim tab pivots and linkages.

 

This is speculation about possible causes, but far better to fix the cause, if at all possible.

 

After that, regular preflight checks of the outer rivet for signs of movement, and prompt action should that occur should be enough: it seems highly unlikely that the setup could go from all rivets secure to all rivets failed in one flight.

 

Beyond that, one 'correct' fix would be to mirror SD025 on the other side of SD024. That way, the forces currently working as a tension (and primarily on the outer rivet) would become a sheer force working on all rivets in the mirrored bracket. Yes, I know, more bits, more weight...

 

 

[rankamateur]

it seems highly unlikely that the setup could go from all rivets secure to all rivets failed in one flight.

One rivet can't move without all the rivets on that half moving so there would be movement for months or years before the heads come right off the rivets. The grey halo around the heads of the rivets would be the first sign.

 

 

[bobcharl]

Anyone know at what hours the known incidents have occurred at?

 

Bob

 

 

[SSCBD]

3/16 aircraft bolt - when in doubt. Fixed for life.

 

 

[JG3]

Anyone know at what hours the known incidents have occurred at?Bob

That was at 1200hrs on my aircraft.

 

I'd hoped to hear from some of those others whom Bill spoke about. I understood there were several, I'm not the only one or the first..... We could learn some clues from what they found..... My experience shows that this can develope into a potentially risky situation, so we need others to post what they found and when.....

 

 

[Ultralights]

after 1000 hrs, had this, no movement, and no loose rivets, though plenty of staining where the anti corrosion solution has been seeping through every join. (the black stuff)

 

 

 

I did have significant wear issues with the brass bushings, some worn completely through. and allowing quite a bit of movement of the hinge bolt. both laterally and vertically. so much so that i modified the elevator tip cap fairings to be easily removable for future inspections.

 

 

 

[IBob]

after 1000 hrs, had this, no movement, and no loose rivets, though plenty of staining where the anti corrosion solution has been seeping through every join. (the black stuff)

 

 

I did have significant wear issues with the brass bushings, some worn completely through. and allowing quite a bit of movement of the hinge bolt. both laterally and vertically. so much so that i modified the elevator tip cap fairings to be easily removable for future inspections.

 

Sounds like good information, Ultralights, but I can't see your pics for some reason. Maybe some app missing on my old laptop. Can anyone else see them?

 

I am currently building an S, and had the elevator temporarily fitted to the stabiliser while fitting the tips. I had one very tight bush on a tip pin, and so a note to do something about that when it comes to final assembly. These are certainly tiny bearing surfaces.

 

 

[JG3]

Those little brass bushes don't last all that long, so long ago I machined teflon bushes and no problems since.

 

 

[IBob]

Those little brass bushes don't last all that long, so long ago I machined teflon bushes and no problems since.

Thank you, JG. I will make that mod before assembly.

 

Are there any other bushes I should be looking at while I'm doing that?

 

 

[fallowdeer]

My top rudder bushing was very worn at 270 hrs.

 

 

[JG3]

Thank you, JG. I will make that mod before assembly.Are there any other bushes I should be looking at while I'm doing that?

Aileron bushes in particular. For those I made the bush with an outswide diameter of 10mm and drilled out the holes in the ailerons to 10mm, then bolted the bush tight to the support bracket. So that the aileron bracket rotates on the outside of the bush, giving a much larger bearing surface of aluminium on teflon, rather than the small area of brass on steel. Haven't shown any wear in many hundreds of hours.

 

 

[IBob]

This is great info, JG. Thank you.

 

 

[rankamateur]

Bob if you do want to run the standard bushes you will note there is a burr in the hole of the bush where they have parted the bush off the stock, tidy that up with a needle file and see how it fits then.

 

 

[IBob]

Bob if you do want to run the standard bushes you will note there is a burr in the hole of the bush where they have parted the bush off the stock, tidy that up with a needle file and see how it fits then.

Thanks, Steve, that would explain the tight bush, which was principally at one end.

 

 

[Marty_d]

Oh dear I have a filthy mind.

 

But seriously guys thanks for the warning. I'll make sure I check for lateral slop on the 701 elevator.

 

 

[rankamateur]

Thanks, Steve, that would explain the tight bush, which was principally at one end.

It is tightest at the flange end of the bush, and don't be shy with the grease when you do your final assembly.

 

 

[rankamateur]

It is tightest at the flange end of the bush, and don't be shy with the grease when you do your final assembly.

And don't worry about Marty, obviously building a 701 is not his only problem.

 

 

[IBob]

And don't worry about Marty, obviously building a 701 is not his only problem.

He's a building a ...what???

 

 

[rankamateur]

He's a building a ...what???

To be fair, I think he is really building it, not assembling a 701 kit.